As is fundamental in the art, the high performance available from modern integrated circuits derives from the transistor matching that automatically results from the fabrication of all of the circuit transistors on the same integrated circuit chip. This matching results from all of the devices on the same chip being fabricated at the same time with the same process parameters. As such, integrated circuits operate in a matched manner over wide variations in power supply voltage, process parameters (threshold voltage, channel length, etc.), and temperature.
However, mere matched operation of the devices on the integrated circuit does not guarantee proper operation, but only means that all devices operate in a matched fashion relative to one another. If, for example, the integrated circuit is manufactured at its "high-current corner" conditions (minimum channel lengths, minimum threshold voltages), all transistors in the chip will have relatively high gains, and will switch relatively quickly; the integrated circuit will thus operate at its fastest, especially at low temperature with maximum power supply voltage applied. Conversely, if the integrated circuit is manufactured at its "low-current corner" (maximum channel lengths, maximum threshold voltages), all transistors in the chip will have relatively low gains and slow switching speeds, and the integrated circuit will operate at its slowest rate, especially at high temperature and the minimum power supply voltage. Accordingly, the factors of processing variations, power supply voltage, and temperature greatly influence the speed and overall functionality of the integrated circuit.
The circuit designer must take into account variations such as these when designing the integrated circuit. For example, the circuit designer may wish to have a certain internal clock pulse to occur very quickly in the critical data path of an integrated memory circuit. However, variations in process, voltage and temperature limit the designer's ability to set the fastest timing of the clock pulse at the slowest conditions (low-current process corner, low voltage, high temperature) without considering that the circuit may be so fast at its fastest conditions (high-current process corner, high voltage, low temperature) that the clock may occur too early or have too narrow a pulse width. An example of such an internal clock pulse is the clock pulse for the sense amplifier in an integrated circuit memory. While additional delay directly increases the access time, incorrect data may be sensed if the sense amp clock occurs too early (i.e., switches too fast).
In addition, many functional circuits internal to an integrated circuit rely upon current sources that conduct a stable current. Examples of such functional circuits include voltage regulators, differential amplifiers, sense amplifiers, current mirrors, operational amplifiers, level shift circuits, and reference voltage circuits. Such current sources are generally implemented by way of field effect transistors, with a reference voltage applied to the gate of the field effect transistor.
As is known in the art, the integrated circuits utilizing such current sources would operate optimally if the current provided by the current source were to be stable over variations in operating and process conditions. However, as is well known in the art, the drive characteristics of MOS transistors can vary quite widely with these operating and process variations. Conventional MOS transistor current sources will generally source more current at low operating temperature (e.g., 0.degree. C.), high V.sub.cc power supply voltage (e.g., 5.3 volts for a nominal 5 volt power supply), and process conditions that maximize drive (e.g., shorter than nominal channel length); conversely, these current sources will source less current at high operating temperature (e.g., 100.degree. C.), low V.sub.cc power supply voltage (e.g., 4.7 volts for a nominal 5 volt power supply), and process conditions that minimize drive current (e.g., longer than nominal channel length). The ratio between the maximum current drive and minimum current drive for such conventional current sources has been observed to be on the order of 2.5 to 6.0. The behavior of circuits that rely on these current sources will therefore tend to vary greatly over these operating and process conditions, requiring the circuit designer to design for a greater operating margin, thus reducing the maximum performance of the integrated circuit.
Many modern integrated circuits are implemented by way of circuits that are controlled by a reference voltage. For example, the current source circuit discussed above is generally implemented as a field effect transistor receiving a reference voltage at its gate. Other circuits, particularly those that control the switching response of logic circuits within modern integrated circuits, may use a series field effect transistor with its gate controlled by a reference voltage to control the switching speed, or slew rate, of the circuit. The reference voltages used in these circuits is produced by a voltage reference circuit, or bias circuit, that is preferably designed to provide a stable reference voltage.
For example, one common technique uses a bias circuit that attempts to compensate for temperature variations. This conventional example relies on the well-known inverse variation of the threshold voltage of a MOS transistor over temperature, by using temperature-dependent threshold voltage variations to produce a temperature-compensating bias voltage. It has been observed, however, that such circuits are not well-suited to compensate for both temperature variations and process parameter variations, since the threshold voltage is itself a process parameter. Variations in the process parameters may thus affect the ability of the circuit to compensate for temperature, such that conventional temperature-compensated bias voltage generating circuits are not well compensated for variations in manufacturing process parameters.
In addition, as described in the above-incorporated application Ser. No. 08/357,664, it has been found to be desirable, for some applications, to provide a reference voltage that tracks variations in the power supply voltage. This tracking reference voltage can allow certain circuit functions, such as output driver slew rate control circuits, to operate in a consistent manner over a wide range of power supply voltages.
It is therefore an object of the present invention to provide a bias circuit for producing a compensated bias voltage that follows variations in power supply voltage and process parameters.
It is a further object of the present invention to provide such a bias circuit that so robustly compensates for variations in power supply voltage and process parameters that temperature variations need not be considered.
It is a further object of the present invention to provide such a bias circuit that compensates for variations in p-channel field effect transistor and process parameters.
It is a further object of the present invention to provide such a bias circuit that compensates for variations in transistor and process parameters for field effect transistors of both p-channel and n-channel types.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.